This invention relates generally to methods and devices for reducing the presence of a biocide in an ionically conductive material, e.g., for use in iontophoretic devices, either during or after the manufacture of the ionically conductive material or an assembly comprising this material. In addition, this invention relates to hydrogels comprising one or more biocides.
A number of diagnostic tests are routinely performed on humans to evaluate the amount or existence of analytes present in blood or other body fluids. These diagnostic tests typically rely on physiological fluid samples removed from a subject, either using a syringe or by pricking the skin.
PCT Publication No. WO 96/00110, published Jan. 4, 1996, describes an iontophoretic apparatus for transdermal monitoring of a target analyte, wherein an iontophoretic electrode is used to move the analyte into a collection reservoir and a biosensor is used to detect the analyte. In U.S. Pat. No. 5,279,543 to Glikfeld, iontophoresis is used to sample a substance through skin and into a receptacle on the skin surface. Glikfeld suggests that this sampling procedure can be coupled with a glucose-specific biosensor or glucose-specific electrodes in order to monitor blood glucose. Additionally, U.S. Pat. Nos. 5,362,307 and 5,730,714 both to Guy, et al. describe sampling devices.
Analytical biosensors have been embraced during the last decade as a means of combining the advantages of electrochemical signal transduction with the specificity inherent in biological interactions. However, two factors that may affect the quality of the data generated by the signal transduction are as follows. First, compounds unrelated to the analyte of interest may enter the analytical system and interact directly with the electrode assembly, leading to signal generation unrelated to the concentration of the analyte or its derivatives. These interfering species may be introduced either during manufacture of the biosensor or during its use. For example, certain compounds present in sample fluid (e.g., acetominophen and uric acid) are electrochemically xe2x80x9cactivexe2x80x9d and are capable of signal generation independent of the specific biological system employed by the biosensor, via a direct interaction with the electrode. Additionally, compounds that may interact at an electrode may have been introduced during manufacturing for specific purposes, such as to provide antimicrobial or antifungal activity (biocides). These interfering species may produce overlapping current signals, thus decreasing the selectivity of the biosensor. Additionally, the compounds may irreversibly bind to the reactive face of the electrode assembly, leading to fouling of the sensing surface and reduced sensitivity.
Several techniques have been employed to minimize the effects of interfering species on electrode function to get around these issues. One technique is to use the lowest polarizing voltage sufficient for the intended reaction. This reduces the current (i.e., electrons) generated by any undesired electrochemical oxidations requiring polarizing voltages higher than what is required for the intended reaction. However, because some enzymatic systems employed in biosensors require voltage levels that do not provide sufficient screening of signals generated by interfering species, the voltage level cannot be decreased below that which allows generation of signals from the interfering species.
A second technique has been to construct membranes or other physical barriers to impede the interfering species from reaching the face of the electrode. The list of films which may be employed includes cellulose acetate, poly(o-phenylenediamine), polyphenol, polypyrrole, polycarbonate, and Nafion(copyright) (E.I du Pont de Nemours and Co., Wilmington Del.) polymer. However, such membranes can be difficult to prepare and may not efficiently attach to the reactive surface of the electrode. There remains a need in the art for methods and devices which provide an efficient reduction of interfering species while maintaining efficient detection of an analyte.
The present invention provides methods and devices for reducing the presence of a compound in an ionically conductive material wherein the presence of the compound interferes with detecting an analyte in the material. By decreasing the level of interfering species present in the ionically conductive material, the present invention increases the percentage of signal that arises from an analyte of interest (or its derivatives) during use of a sampling device. In one aspect of the present invention, the reduction in interferant signal is achieved by selectively adsorbing the interfering compound from the ionically conductive material before the compound can reach the sensor means and generate a signal. In a second aspect of the invention, the interfering species are reduced by polymerizing an interfering compound to form an electrochemically-inactive but permeation selective barrier at the reactive face of the sensor means. The permeation selective characteristics of the polymer barrier can provide the added benefit of reducing signals generated from interferants other than the species being polymerized. Because the aforementioned permeation selective barrier is created on the reactive face of the sensor means in situ rather than prior to construction of the collection assembly, the present invention provides efficient means for manufacturing collection assemblies that use this method for reducing the presence of an interferant compound.
Accordingly, it is a general object of the invention to provide a method for reducing the presence of a compound in an ionically conductive material wherein the presence of the compound interferes with detecting an analyte in the material. In one embodiment, the method includes placing the material containing the compound in contact with at least one component of a device used for detecting the analyte, wherein the component is partially permeable to the compound. The component and the compound are contacted under conditions that allow the compound to migrate out of the material and into the component, thus reducing the presence of the compound in the material. In the present invention, the component is preferably composed of a polyurethane-like material or a polyester-like material.
In another embodiment of the present invention, the presence of an interfering compound is reduced essentially as follows. The ionically conductive material containing the interfering compound is placed in contact with a reactive face of a sensor element (for example, a sensor electrode). The ionically conductive material and the sensing element are arranged such that when a current is flowing to the sensing element, the current flows through the ionically conductive material containing the compound. The sensor element is then activated to provide an electrical current for a period of time and under conditions sufficient to polymerize the compound on the reactive face of the sensor. Previous approaches for forming permeably selective films on electrodes required that the film was formed ex situ, that is before use, and the present invention demonstrates that the permeably selective barrier can be formed in situ. In the present invention, a preferred group of polymerizable interferant compounds are phenolic compounds, for example the p-hydroxybenzoic acid esters commonly referred to as xe2x80x9cparabens.xe2x80x9d
In a further embodiment of the invention, a method of forming a permeation-selective barrier on an electrode face in situ is described, the method comprising the steps of a) formulating an ionically conductive material comprising a phenolic compound capable of polymerizing under the influence of an electrical current, b) placing the material in contact with a reactive face of a sensing electrode such that when current is flowing to the electrode current flows through the material, and c) activating the electrode to provide an electrical current for a period of time and under conditions sufficient to polymerize the compound on the reactive face of the sensor and form a permeation-selective barrier. In the present invention, a preferred group of phenolic compounds are the p-hydroxybenzoic acid esters commonly referred to as xe2x80x9cparabens.xe2x80x9d
In another embodiment of the present invention, a collection assembly for use in a sampling system is described. The collection assembly is comprised of a collection insert layer containing an ionically conductive material, wherein the ionically conductive material contains a compound that will polymerize on the reactive face of a sensor element placed adjacent to the ionically conductive material. Also described is a method of manufacturing a collection assembly The method of manufacture of the collection assembly comprises the steps of a) forming the ionically conductive medium containing the interfering compound, b) contacting one surface of the ionically conductive medium with a mask layer composed of a material that is substantially impermeable to the selected analyte or derivatives thereof, and c) contacting a second surface of the ionically conductive medium with a retaining layer to form the collection assembly.
In a further embodiment of the present invention, an autosensor assembly for use in a sampling system is described. The autosensor assembly is comprised of a) a collection insert layer containing an ionically conductive medium, an enzyme capable of reacting with an analyte to produce hydrogen peroxide, and a phenolic compound which will polymerize under an electric current; and b) a sensor element in operative contact with the collection insert layer, positioned such that the phenolic compound can react electrochemically with the reactive face of the sensor element to provide a selectively permeable barrier at an interface between the sensor element and the collection insert layer.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.